MCP33131 datasheet(32/54 Pages) MICROCHIP

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DS20006122A-page 32

 2018 Microchip Technology Inc.

MCP33131/MCP33121/MCP33111-XX

6.3

ADC Input Driver Selection

The noise and distortion of the ADC input driver can

degrade the dynamic performance (SNR, SFDR, and

THD) of the overall ADC application system. Therefore,

the ADC input driver needs better performance specifi-

cations than the ADC itself. The data sheet of the driver

typically shows the output noise voltage and harmonic

distortion parameters.

Figure 6-3

shows

simplified

system

noise

presentation block diagram for the front-end driver and

ADC.

FIGURE 6-3:

Simplified System Noise

Representation.

• Unity-Gain Bandwidth:

An input driver with higher bandwidth usually results in

better overall linearity performance. Typically, the driver

should have the unity-gain bandwidth greater than 5

times the -3 dB cutoff frequency of the anti-aliasing fil-

ter:

EQUATION 6-2:

BANDWIDTH

REQUIREMENT FOR ADC

INPUT DRIVER

where, fB = -3 dB bandwidth of RC anti-aliasing filter as

shown in Figure 6-3.

• Distortion:

The nonlinearity characteristics of the input driver

cause distortions in the ADC output. Therefore, the

input driver should have less distortion than the ADC

itself. The recommended total harmonic distortion

(THD) of the driver is at least 10 dB less than that of the

ADC:

EQUATION 6-3:

RECOMMENDED THD

FOR ADC INPUT DRIVER

• ADC Input-Referred Noise:

When the ADC is operating with a full-scale input

range, the ADC input-referred RMS noise for a

single-ended input configuration is approximated as

shown in Equation 6-4.

EQUATION 6-4:

ADC INPUT-REFERRED

NOISE

• Noise Contribution from the Front-End Driver:

The noise from the input driver can degrade the ADC’s

SNR performance. Therefore, the selected input driver

should have the lowest possible broadband noise den-

sity and 1/f noise. When an anti-aliasing filter is used

after the input driver, the output noise density of the

input driver is integrated over the -3 dB bandwidth of

the filter.

Equation 6-5 shows the RMS output noise voltage cal-

culation using the RC filter’s bandwidth and noise den-

sity (eN) of the input driver. GN in Equation 6-5 is the

noise gain of the driver amplifier and becomes 1 for a

unity gain buffer driver.

EQUATION 6-5:

NOISE FROM FRONT-END

DRIVER AMPLIFIER

where eN is the broadband noise density (V/√Hz) of the

front-end driver amplifier and is typically given in its

data sheet. In Equation 6-5, 1/f noise (eNFlicker) is

ignored assuming it is very small compared to the

broadband noise (eN).

For high precision ADC applications, the noise

contribution from the front-end input driver amplifier is

typically constrained to be less than about 20% (or 1/5

times) of the ADC input-referred noise as shown in

Equation 6-6:

EQUATION 6-6:

RECOMMENDED ADC

INPUT DRIVER NOISE

Using Equation 6-4 to Equation 6-6, the recommended

noise voltage density (eN) limit of the ADC input driver

is expressed in Equation 6-7:

ADC Input Driver

ADC

VN_ADC Input-Referred Noise

VN_RMS_Driver Noise

RC

---------------



(Hz)

BWInput Driver  5 x f

for a single-pole RC filter

THDInput Buffer ≤ THDADC -10

(dB)

VN_ADC Input-Referred Noise

(V)

VREF

--------------

=10

SNR

-----------

–

VN_RMS_Driver Noise

-------

 f



(V)

VN_ADC Input-Referred Noise

VN_RMS_Driver Noise

---



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